Python If-then command

I have a code as follows

GU_ES3 ['Qnd'] = GU_ES3 ['Qnd,hw_m2'] * GU_ES3 ['UPOR']

What I found out is, that some valus in GU_ES3 ['UPOR'] are blank. And in this scenario, I'd like to call some other column, e.g. GU_ES3 ['NPOR'], but only in this case when values in column GU_ES3 ['UPOR'] are 0 or NaN.

Can you help me?